Advances in Sintering Technologies for SiC Ceramics: Mechanisms, Challenges, and Industrial Applications

2.1. Pressureless Sintering Pressureless sintering (PS) is a commonly used and cost-effective method for producing dense SiC ceramics. It can be broadly classified into solid-state sintering and liquid-phase sintering (LPS) according to the type of sintering aids employed [22,23]. The choice of sintering aids plays a crucial role in determining the densification mechanisms and has a substantial influence on the final SiC microstructure, including factors such as grain size and the composition of the grain boundary phase. Consequently, the overall mechanical, thermal, and chemical properties of the fabricated SiC ceramics vary depending on the sintering method. Solid-state sintering primarily relies on boron-carbon-based additives to facilitate densification, while liquid-phase sintering makes use of oxide systems that create transient liquid phases, enabling densification at relatively lower temperatures. This section focuses on solid-state sintering due to its simplicity and widespread industrial application, with a particular emphasis on recent progress and challenges in this area. 2.1.1. Solid-State Sintering Solid-state sintering of SiC ceramics utilizes boron-carbon (B/C) series additives, including B, B + C, and B₄C + C, Al₃BC₃ to facilitate densification. The densification process is governed by two complementary mechanisms. Boron segregates at grain boundaries, thereby reducing the interfacial energy (γ_GB). Carbon removes the native SiO₂ layer on SiC particles through carbothermal reduction, which enhances the surface energy (γ_SV). The combined effect of these sintering aids lowers the γ_GB/γ_SV ratio to a subcritical value, rendering densification thermodynamically advantageous [38]. In solid-state sintering, sintering temperatures usually exceed 1900 ℃, guaranteeing sufficient diffusion activity. Recent studies have explored the effects of different additives and processing parameters on the thermal and mechanical properties of SS-SiC ceramics [22,32,33,34,39,40,41]. Zhao et al. [22] investigated the addition of β-SiC nanoparticles (β-SiC_np) to enhance the thermal conductivity of solid-state sintered SiC ceramics. The introduction of β-SiC_np resulted in the homogeneous encapsulation of α-SiC particles, filling microstructural voids and enabling grain growth. These structural changes promoted longer phonon mean free paths, improving thermal conductivity. The optimized β-SiC_np additions achieved >100 W·m⁻¹·K⁻¹ in thermal conductivity, a marked improvement over conventional formulations. Additionally, for the aerospace applications of SiC ceramics, maximizing specific stiffness is critical to minimizing structural weight. Yeom et al. [42] investigated the effect of B₄C content on specific stiffness and found that the specific stiffness reached a maximum of 144.4 × 10⁶ m²·s⁻² at a B₄C content of 20 wt.%. Meanwhile, the study also revealed that when the B₄C content was 10 wt.%, the flexural strength (~520 MPa) and fracture toughness (~4.2 MPa·m^1/2) reached their highest values, while the thermal conductivity decreased due to the formation of amorphous intergranular phases when the B₄C content exceeded 20 wt.% (Figure 3). These observations emphasize the trade-offs between mechanical strength, fracture toughness, and thermal performance when optimizing additive contents. In addition, SS-SiC ceramics find applications in those requiring intermediate mechanical performance and wear resistance, such as kiln furniture, ballistic armor, and structural components for mechanical systems [43]. SiC ceramic ballistic armor has received significant attention from scholars in the defense sector. Dong et al. [43] suggest that the key challenge in developing SiC ballistic armor lies in optimizing the types and proportions of raw materials and additives to sinter silicon carbide ceramics. This optimization is crucial for balancing the hardness and fracture toughness of the ceramics, enabling them to withstand both high-penetration impacts and the shocks from multiple weapon types. The future development of ballistic armor is expected to focus on lightweight, composite structures with high hardness and toughness. Despite its simplicity, the solid-state sintering of SiC ceramics encounters multiple challenges. The high sintering temperature (~1900–2100 ℃) required for effective densification is associated with substantial energy consumption and potential grain coarsening, leading to degraded mechanical properties [22,34]. As a result, expanding their industrial applicability requires advancements in additive optimization and process innovations. To illustrate, researchers have proposed utilizing multi-step heating profiles to limit grain coarsening and optimize thermal gradients during sintering [44]. Future work may also involve integrating solid-state sintering with secondary phases, such as SiC whiskers, graphene nanoplatelets, or other reinforcements, to enhance fracture toughness and thermal conductivity [22]. In summary, although pressureless solid-state sintering remains a fundamental technique in SiC fabrication due to its simplicity and scalability, the high sintering temperature limitations create challenges. Continued innovation in sintering aid chemistry and process design is essential for addressing these barriers and meeting the needs of emerging high-performance applications. 2.1.2. Liquid-Phase Sintering Aiming at the high preparation temperature of SS-SiC ceramics, liquid-phase sintering (LPS) provides a low-temperature sintering approach. It mainly achieves the densification of silicon carbide ceramics at a lower temperature (~1800 ℃) by using low-melting-point sintering aids such as SiO₂, Al₂O₃, AlN, and rare-earth oxides [23]. These additives play a crucial role in forming transient liquid phases during sintering, which facilitate mass transport and densification through enhancing grain boundary diffusion and particle rearrangement [13,45,46,47,48,49]. However, although LPS-SiC ceramics achieve near-theoretical densities (>98%), their mechanical and thermal properties at elevated temperatures are often compromised by the presence of amorphous intergranular phases. These films tend to soften and weaken at high service temperatures (>1200 ℃), posing challenges for LPS-SiC ceramics, particularly in long-term high-temperature applications where thermal stability is essential [50,51,52,53]. Therefore, enhancing the high-temperature resistance of LPS-SiC ceramics has become a main focus in this field. Recent studies have demonstrated the effectiveness of these approaches in improving LPS-SiC properties. Seo et al. [52] reported that optimizing rare-earth oxide additives (such as Y₂O₃ and Sc₂O₃) during LPS facilitated the formation of crystalline intergranular phases, achieving maintained 93% of its room temperature (RT) strength up to 1600 ℃, a marked improvement compared to conventional LPS-SiC with amorphous boundaries. Similarly, studies have shown that by using an extremely small amount of sintering aids (2000 ppm Y₂O₃), it is possible to maintain an extremely high flexural strength (981 ± 128 MPa) at 2000 ℃ by reducing the residual amorphous phases [53]. Additionally, since the viscosity of Si-O-C glass is strongly influenced by the contents of Al, O, and N, AlN and various rare-earth oxides are also added as sintering aids to LPS-SiC [54]. SiC ceramics exhibit exceptional chemical resistance and thermal stability in high-temperature corrosive environments, making them promising candidates for applications such as light-water reactor components. A recent comparative study [55] evaluated the hydrothermal corrosion resistance of SS-SiC and LPS-SiC ceramics under extreme conditions (360 ℃ and 18.6 MPa). The results revealed that SS-SiC exhibited concentrated void formation around residual sintering additives (e.g., B and C). These voids functioned as stress concentrators, accelerating crack propagation and delamination in specimens with pre-existing flaws. In contrast, in the absence of prefabricated cracks, LPS-SiC ceramics experienced higher weight loss due to the hydration of intergranular amorphous phases, which compromised their structural integrity (Figure 4). These findings emphasize the importance of optimizing the composition and purity of sintering aids to reduce the vulnerability of LPS-SiC to hydrothermal attack. Despite its advantages, liquid-phase sintering has its challenges. The presence of amorphous intergranular films limits the high-temperature mechanical reliability of LPS-SiC ceramics. Moreover, shrinkage and residual stress during cooling require precise control to ensure dimensional accuracy, particularly for large-scale components. Additionally, the susceptibility of amorphous grain-boundary phases to hydrothermal degradation in aggressive environments remains a major concern. To address these issues, future research should focus on developing new sintering aids that either minimize amorphous phase formation or promote their crystallization during sintering. Furthermore, combining liquid-phase sintering with advanced post-processing methods, such as isostatic pressing, could potentially enable the production of large-scale components with superior mechanical properties. 2.2. Reaction-Bonded Sintering Reaction-bonded sintering (RB-SiC) is one of the earliest industrial sintering processes developed for silicon carbide ceramics, characterized by its low sintering temperature, short processing time, and close to the final shape capability [5,8]. In the traditional reaction-bonded silicon carbide process, ceramics are manufactured by blending silicon carbide powder with a small amount of carbon powder, followed by high-temperature infiltration of molten silicon. This reaction forms a dense SiC matrix through an in-situ process [56]. Due to its scalability and cost-effectiveness, RB-SiC has been widely adopted in industrial applications. However, with increasing demands for more complex geometries and high-performance components, traditional RB-SiC no longer meets the stringent requirements of modern engineering. Current research on RB-SiC is focused on advanced forming techniques such as injection molding, tape casting, and additive manufacturing, as well as brazing technologies and the effects of additives and particle size distribution on the microstructure and properties of the final ceramics [24,56,57,58,59,60,61,62,63,64,65]. One critical limitation of traditional RB-SiC is the presence of residual free silicon. The incomplete reaction between infiltrated silicon and carbon results in unreacted free silicon, which compromises the material’s mechanical strength, thermal stability, and maximum operating temperature. As such, RB-SiC is often limited to applications below 1380 ℃ [30,31,66]. Reducing the free silicon content has thus become a central objective in RB-SiC research [67,68,69,70,71,72,73]. Strategies employed to reduce free silicon content include optimizing the pore characteristics and carbon content of SiC/C preforms, enhancing silicon infiltration efficiency, and employing customized sintering conditions. Park et al. [63] demonstrated that using high-density diamond as the primary carbon source allowed for optimal pore channel design, which improved silicon infiltration into high-carbon SiC/C preforms. This resulted in RB-SiC ceramics with bending strengths exceeding 200 MPa at 1500 ℃ (Ar). These advancements highlight that careful control of preform properties and sintering dynamics is critical to improving the performance of RB-SiC ceramics. Low-cost, high-performance RB-SiC with reduced free silicon content represents a promising direction for industrial-scale applications. In advanced applications requiring lightweight and complex geometries, such as large-scale structural components, heat exchangers, and catalyst carriers, traditional forming methods for RB-SiC have limitations in producing intricate designs. Additive manufacturing (AM), combined with liquid silicon infiltration, has emerged as a transformative technology enabling the fabrication of complex RB-SiC ceramics with unprecedented design freedom and enhanced material utilization. This approach allows the integration of intricate features, such as internal channels and lattice structures, which are challenging to achieve using conventional methods [1,58,59,74,75]. Figure 5 illustrates RB-SiC specimens fabricated using additive manufacturing technologies, showcasing their potential for lightweight and complex structural designs. Recent advancements in additive manufacturing for RB-SiC focus on material and process optimization. Wang et al. [74] explored the shape retention and defect control of silicon carbide whiskers (SiC_w) during de-binding in light-curing molding techniques. By optimizing de-binding and sintering protocols, they successfully developed SiC whisker-reinforced RB-SiC ceramics (SiC_w/RBSiC) with bending strengths of 352.2 MPa and Vickers hardness of 17.54 GPa. Pelanconi et al. [75] employed polyamide powder bed fusion technology to fabricate green compacts, followed by ceramic precursor polymer infiltration, pyrolysis, and liquid silicon infiltration, yielding helical SiC ceramics with a maximum compressive strength of 24.7 ± 2.2 MPa, a skeletal density of 3.173 ± 0.022 g/cm³, and a relative density of 93.5%. The results demonstrate the effectiveness of this approach in fabricating complex ceramic structures for engineering applications such as heat exchangers and catalyst supports. These studies fabricate SiC green compacts by sintering/melting binders in raw materials and obtain sintered bodies through debinding and subsequent sintering processes, which may lead to deformation or shrinkage. However, SiC lacks a molten phase under normal atmospheric conditions, making it extremely challenging to prepare SiC products in a one-step process using direct additive manufacturing methods such as Laser Powder Bed Fusion (L-PBF) or Selective Laser Sintering/Melting (SLS/SLM) [76]. How to simplify the process and fabricate high-performance SiC ceramics will become one of the research priorities in the field of SiC additive manufacturing. Meyers et al. [77] investigated selective laser sintering using SiC and Si powders as starting materials, where silicon is melted and resolidified to bond the original silicon carbide particles. However, to fabricate RB-SiC components with higher performance, researchers still employed post-processing techniques such as resin impregnation, pyrolysis, and liquid silicon infiltration. In addition, RB-SiC ceramics have been applied in the photovoltaic field. High-temperature carriers produced from silicon carbide, such as boat supports, uniform flow plates, silicon wafer boats, etc. (Figure 6, the picture is sourced from Science X (Huangshi, Hubei) New Material Technology Co., Ltd.), are increasingly replacing quartz and becoming crucial high-temperature devices in the thermal processing/thermal coating processes of photovoltaic production. Their complex geometries pose higher requirements for the additive manufacturing process. Such developments affirm the potential of additive manufacturing in producing high-performance RB-SiC composites with controlled microstructures. Additionally, customizing printing parameters, raw material formulations, and sintering systems has proven critical to achieving defect-free, high-density RB-SiC components. 2.3. Recrystallization Sintering Recrystallization sintered silicon carbide (R-SiC) is a unique sintering method that primarily employs SiC powder as the raw material. During the sintering process, evaporation, condensation, and recrystallization occur within a high-temperature and controlled-pressure protective atmosphere, leading to the formation of a sturdy SiC sintered body. In contrast to other sintering methods, R-SiC does not rely on external additives or binders to achieve densification. Instead, the recrystallization mechanism generates a sintered structure with minimal dimensional shrinkage. Throughout the sintering process, the distance between the centers of large SiC particles remains nearly constant, thereby preserving the overall volume stability of the material [14,37,78]. A notable feature of R-SiC is the presence of continuous gas pores within its structure. These pores are formed as a result of the limited densification and interconnected porosity during the recrystallization process. This distinctive pore structure confers R-SiC with unique properties, making it highly suitable for applications in high-temperature flue gas filtration, catalyst carriers, and diesel particulate filters [14,35,36]. Recent research has focused on optimizing the sintering parameters of R-SiC and investigating their impact on physical and mechanical properties, with particular emphasis on pore structure, permeability, and mechanical strength [26,35,76,77]. For example, Wang et al. [79] utilized the Dinger-Funk particle packing model to design optimized particle size distributions for SiC powders. This facilitated the fabrication of an R-SiC ceramic membrane with remarkable permeability. Figure 7 illustrates the relationship between the structural evolution of silicon carbide ceramic membranes and different sintering temperatures, as well as its sintering mechanism. The prepared membrane achieved a permeability of 1210 L·m⁻²·h⁻¹·bar⁻¹, demonstrating excellent performance in carbon black wastewater treatment. This study highlights the crucial role of particle packing and sintering neck formation in adapting the transport properties of R-SiC for filtration applications. In another study, Yu et al. [14] introduced Al₄SiC₄ as a sintering additive to prepare R-SiC honeycomb ceramics via a two-step sintering process. The research explored the solubility behavior of AlN at various temperatures and its impact on sintering neck parameters and the mechanical properties of the honeycomb structures. Figure 8 shows the micro-morphology and the diagram of the high-temperature reaction mechanism of the nitrided-recrystallized SiC ceramics prepared at different sintering temperatures. The results revealed that uniform sintering necks with optimal connectivity were crucial for enhancing mechanical strength while maintaining sufficient porosity. The addition of Al₄SiC₄ enabled precise control over the pore structure and mechanical properties of the R-SiC ceramics, making them suitable for tailored industrial applications. Through continuous advancements in pore structure optimization and sintering techniques, R-SiC ceramics are expected to make significant contributions to industries that demand high-temperature filtration, catalysis, and environmental protection. 2.4. Pressure-Assisted Sintering 2.4.1. Hot-Pressing Sintering Hot-pressing sintering (HP) is a mechanical pressure-assisted sintering method (Figure 9). In this process, ceramic powders or green compacts are placed inside a die cavity and then subjected to uniaxial pressure at elevated temperatures. The external pressure enhances the densification process by promoting particle rearrangement and reducing pore size, allowing for the production of fine-grained and uniform microstructures within a relatively short sintering period. This method exhibits several advantages, including yielding superior mechanical properties, reducing both the sintering time and temperature required, lowering the requirements for sintering aids, and improving the high-temperature performance of the final material [80,81,82]. However, there are limitations to hot-pressing sintering. Due to the constraints imposed by uniaxial pressing, this method is generally applicable only to simpler geometries. For example, it is well-suited for fabricating disk-shaped components, as well as other flat or axisymmetric parts such as chip carriers and sealing rings [9,25]. Hot-pressing sintering is extensively employed for the fabrication of high-melting-point ceramic composites, covering carbides and borides [83,84,85,86]. However, the strong covalent bonding within SiC poses a challenge in achieving complete densification without the aid of sintering additives, even when subjected to the high pressures and temperatures characteristic of hot pressing. As an example, Lee et al. [87] managed to prepare high-purity SiC ceramics via hot pressing at 2350 ℃ and 50 MPa, yet the relative density obtained was merely 92%. This clearly illustrates the difficulty in attaining full densification in SiC ceramics without sintering aids, which presents obstacles for industrial-scale production. To overcome these limitations, researchers have delved into the utilization of sintering aids to promote densification and enhance the mechanical properties of hot-pressed SiC ceramics. Tao et al. [88] employed aluminum powder, boron powder, and carbon black as sintering aids to produce silicon carbide nanofiber-reinforced silicon carbide (SiC_nf/SiC) composites through slurry impregnation and hot-pressing sintering. Under sintering conditions of 1950 ℃ and 30 MPa, the in-situ formation of an Al₈B₄C₇ liquid phase substantially facilitated the densification process, yielding a bending strength of 548 MPa and a fracture toughness of 15.86 MPa·m½. Likewise, liquid-phase sintering aids like Al₂O₃, Y₂O₃, and metallic Mg have also been demonstrated to foster densification in hot-pressing sintering by generating transient liquid phases that lower grain boundary energy and assist in grain rearrangement [9,89]. In addition to liquid-phase sintering aids, recent investigations have showcased the efficacy of employing smaller amounts of highly dispersed sintering aids to obtain high-purity hot-pressed SiC ceramics. As a representative example, boric acid and fructose have been utilized as sintering aids in high-pressure sintering to yield SiC ceramics with a residual boron content as low as 556 ppm and a relative density of 99.5% [25]. Owing to the low sintering temperature during HP sintering and the fact that the applied axial pressure suppresses grain growth, the grain size of high-purity ceramics is relatively small (Figure 10). This approach not only reduces the amount of sintering additives required but also minimizes the impact of residual impurities on the material’s high-temperature performance, making it a promising strategy for producing high-purity SiC ceramics. Despite its merits, hot-pressing sintering requires considerable equipment and process constraints. In particular, it necessitates meticulous control of temperature, pressure, and mold design. These stringent requirements restrict its applicability to relatively straightforward geometries and small-scale production. Moreover, the high pressures and temperatures implicated in hot pressing demand the employment of sturdy and costly equipment, which can inflate production costs. Consequently, the application of hot-pressed SiC ceramics is frequently confined to high-value components where exceptional mechanical properties and thermal stability are imperative. 2.4.2. Gas Pressure Sintered and Hot Isostatic Pressing Gas Pressure Sintering (GPS) and Hot Isostatic Pressing (HIP) are sintering processes for ceramic powders, green compacts, or sintered bodies. These processes use gas as a pressure medium to apply isotropic pressure during high-temperature and high-pressure sintering, achieving densification through the combined effect of high temperature and pressure. Both GPS and HIP are important for eliminating porosity, refining grain size, and increasing density, which makes them particularly suitable for producing complex-shaped ceramic components [90,91,92,93,94,95,96]. In Gas Pressure Sintering, N₂ is mainly used as the pressure medium and reactive gas. The sintering pressure is relatively low, so it is commonly used in the production of nitride ceramics or ceramics containing nitride compounds [96,97]. Research on the gas pressure sintering of SiC ceramics often relates to SiC composite ceramics or ceramics with reinforcing phases formed in situ by nitridation [92,97]. For example, Wu et al. [98] used Al₄SiC₄ as the raw material and produced a new type of AlN-SiC-C composite ceramic by sintering at 1700 ℃ under a nitrogen gas pressure of 20 atmospheres. The product had a relative density of 75.8%, a volume density of 2.20 g/cm³, and a flexural strength of 120.9 MPa. The material had a cellular microstructure composed of interlocking worm-like SiC and C particles along with AlN ceramic boundaries (Figure 11). On the contrary, Hot Isostatic Pressing (HIP) generally uses an inert gas like Ar as the pressure medium, which does not participate in reactions. HIP applies higher sintering pressures and is usually used for densifying ceramics or metal/ceramic composite materials [94,99,100,101]. For instance, Lv et al. [93] used nanoparticulate β-SiC powder, Si powder, C powder, and microparticulate TiH₂ powder as raw materials to in situ synthesize SiC-32%TiC composite ceramics through a hot isostatic pressing process at 1600 ℃, 120 MPa for 4 h. The resulting SiC-32%TiC composite ceramics showed the best densification, hardness, flexural strength, and good fracture toughness, with a density of 98.7%, hardness of 21.2 GPa, flexural strength of 428 MPa, and fracture toughness of 5.5 MPa·m^1/2 However, the equipment costs for Gas Pressure Sintering and Hot Isostatic Pressing are high. Especially, it is difficult to manufacture equipment for ultra-high temperatures and pressures, which further limits their widespread industrial application. Controlling equipment costs and integrating with additive manufacturing and computer simulations to accurately predict the effects of factors such as pressure, temperature, and time on material density and porosity changes could be key directions for the future development of GPS and HIP processes. This would enable the optimization of process schemes before actual production, reduce trial and error, and lower research and development costs. 2.4.3. Spark Plasma Sintering Spark Plasma Sintering (SPS) is an advanced powder consolidation technique that has gained significant attention in the field of ceramic materials due to its ability to achieve rapid densification and fine-grained microstructures (Figure 12). The SPS process involves the application of high-energy, rectangular direct current pulses, which generate resistive heating and activate the powder surface through the formation of spark plasma in the interparticle gaps. This unique combination of electrical and thermal effects accelerates densification and promotes the formation of strong interparticle bonds, making SPS a highly efficient sintering method [27,102,103,104,105]. Despite its advantages, the densification temperature of silicon carbide (SiC) ceramics in SPS remains above 2000 ℃ in the absence of sintering aids, primarily due to the strong covalent bonding in SiC that limits atomic diffusion [103]. To address this, recent research has focused on exploring various sintering aids. These include oxide-based liquid phase additives like Al₂O₃ and Y₂O₃, which form transient liquid phases during sintering to facilitate grain boundary sliding and improve densification [27,104]; B-C system solid phase additives such as boron and carbon-based ones that reduce grain boundary energy and enhance diffusion for densification at lower temperatures [105,106,107]; MAX phases like Ti₃SiC₂, which are ternary carbides and nitrides with unique layered structures that promote grain refinement and improve mechanical properties [89,108]; and graphene and carbon nanostructures, whose incorporation not only enhances the electrical conductivity of SPS-SiC ceramics but also improves fracture toughness and thermal conductivity [109]. In addition to sintering aids, researchers have also investigated the influence of sintering parameters, including the sintering regime, powder layer thickness, and applied pressure, on the densification behavior and physical properties of SPS-SiC ceramics. Optimizing the heating rate and holding time can affect grain growth and pore elimination, thereby improving the mechanical and thermal properties of the final product [110,111,112]. Moreover, precise control of powder thickness during sintering has been shown to impact the uniformity of densification, particularly in large-scale components. Another promising application of SPS technology is in the bonding of SiC ceramics. Employing the localized heating and high pressure essential to SPS, researchers have developed techniques for joining SiC components. This enables the fabrication of complex ceramic structures that are otherwise challenging to produce using conventional methods. This approach offers a novel strategy for manufacturing advanced SiC devices, such as heat exchangers, turbine components, and structural parts for aerospace applications [113,114]. The ability to bond SiC ceramics with minimal residual stress and high joint strength further highlights the versatility and potential of SPS technology. In summary, Spark Plasma Sintering represents a cutting-edge sintering technology with significant potential for the fabrication and bonding of SiC ceramics. While challenges remain in reducing the densification temperature and optimizing sintering parameters, advancements in sintering aids and process control are paving the way for the development of high-performance SPS-SiC ceramics. Future research should focus on exploring novel sintering aid systems, integrating SPS with other advanced techniques, such as additive manufacturing [115], and scaling up the process for industrial applications, particularly in the fields of energy, aerospace, and electronics. 2.4.4. Oscillatory Pressure Sintering Oscillatory pressure sintering (OPS) is an advanced pressure-assisted sintering technique. In this technique, oscillatory pressure is applied at a specific frequency during the ceramic sintering process (Figure 13). This creates a dynamic pressure environment. Compared to conventional constant-pressure sintering methods such as hot pressing and spark plasma sintering, OPS introduces additional driving forces for densification, resulting in several notable advantages. These include a reduction in sintering temperature, improved material density and densification rate, and enhanced mechanical properties of the sintered ceramics [28,116,117,118,119]. The densification mechanisms in OPS combine traditional processes—such as grain boundary diffusion, lattice diffusion, and evaporation-condensation driven by surface energy—with unique mechanisms induced by oscillatory pressure. These new mechanisms include particle rearrangement, grain boundary sliding, plastic deformation, grain movement due to deformation, and the expulsion of pores [118]. At the same sintering temperature, OPS-SiC ceramics exhibit smaller average grain sizes, indicating that in addition to the lower sintering temperature, the oscillatory pressure applied during OPS also inhibits grain growth. Furthermore, compared with HP-SiC, the grain size-relative density curve of OPS-SiC shifts toward higher relative density, which also demonstrates that oscillatory pressure can effectively suppress grain growth. The dynamic pressure environment created by OPS thus accelerates sintering kinetics and improves the microstructural homogeneity of the final product. OPS technology has been particularly effective in the sintering of SiC-based composite materials reinforced with one-dimensional (e.g., silicon carbide whiskers, SiC_w) and two-dimensional (e.g., graphene nanoplatelets (GNP_s)) reinforcements [6,116,120,121]. Notably, researchers have successfully used OPS to fabricate SiC-GNP composites with higher density and superior mechanical properties compared to those produced under constant-pressure sintering at the same temperature [120]. The incorporation of reinforcements such as SiC_w and GNP_s enhances the strength and toughness of SiC-based ceramics through mechanisms such as crack deflection, crack bridging, interfacial bonding between reinforcements and the SiC matrix, and strong bonding at SiC/GNP_s and SiC/SiC_w interfaces (Figure 14) [116]. These mechanisms not only improve the fracture toughness of the composite but also contribute to its ability to resist crack propagation under mechanical stress. In addition to mechanical improvements, OPS also positively affects the tribological properties of SiC-based ceramics. Researchers have investigated the effects of external factors, such as applied load and sliding speed, on the wear behavior of SiC ceramics fabricated using OPS. It was observed that the addition of GNP_s and SiC_w altered the wear mechanism from brittle fracture-dominated abrasive wear to adhesive wear and micro-cutting, reducing wear rates. This transition in wear behavior highlights the ability of OPS to enhance both the strength and wear resistance of SiC-based ceramics, making them suitable for demanding applications [122]. The unique advantages of OPS technology make it highly promising for applications in fields such as aerospace, where components like engine blades and rotors require exceptional strength, toughness, and thermal stability at high service temperatures. However, the widespread industrial adoption of OPS is currently limited by high equipment costs, challenges in scaling up the process, and the complexity of manufacturing large-sized or intricately shaped SiC components. Future developments in OPS technology should focus on reducing sintering costs through innovations in equipment design and process optimization. Additionally, the development of scalable OPS systems capable of fabricating large and complex SiC structures will be critical for expanding its industrial applications. In summary, Oscillatory Pressure Sintering represents a novel and effective method for producing high-performance SiC-based ceramics. By leveraging dynamic pressure mechanisms to enhance densification, microstructural uniformity, and mechanical properties, OPS has the potential to address the stringent requirements of advanced engineering applications. Continued advancements in process scalability and cost reduction will be essential for unlocking the full potential of OPS in industrial manufacturing. 2.5. Flash Sintering Flash Sintering (FS) is a groundbreaking electric field-assisted rapid sintering technique first introduced by Cologna et al. in 2010 [123]. The core principle of this method involves applying an external electric field to the ceramic sample, generating intense Joule heating effects within the material (Figure 15). This enables rapid densification, often accompanied by a characteristic “flash” phenomenon, representing a sudden surge in current and temperature. Flash sintering has acquired significant attention due to its ability to achieve ultra-fast sintering rates and substantial energy savings compared to conventional sintering methods. One of the most notable advancements in flash sintering was achieved by Gibson et al. [124], who successfully demonstrated pressureless flash sintering of SiC ceramics for the first time using B and C as sintering aids. This approach produced SiC ceramics with a density of 94.4%, an average grain size of 5.9 ± 0.5 μm, and a Vickers hardness of 24.7 ± 0.5 GPa. Compared to traditional pressure-assisted sintering, flash sintering reduced the processing time by over six hours and lowered the furnace temperature by 700 ℃, highlighting its potential for energy-efficient and time-saving ceramic manufacturing. In another study, Shin et al. [125] utilized high-purity silicon scrap derived from semiconductor manufacturing waste, combined with Y₂O₃ as a sintering aid, to fabricate β-SiC ceramics via flash sintering. The addition of Y₂O₃ enabled sintering at a lower furnace temperature of 1133 ℃ while enhancing the densification of the β-SiC ceramics. This study not only demonstrated the feasibility of recycling industrial byproducts for high-performance ceramic fabrication but also highlighted the role of sintering aids in optimizing flash sintering parameters. Their research also found that the porosity in the area near the surface of FS-SiC ceramics is higher than that in the interior, while the grain size is smaller than that in the interior region. This indicates that a gradient temperature field occurs in the green compact during sintering, with the position close to the electrode being lower in temperature. The inhomogeneous microstructure poses challenges for the improvement of sintering processes and the application of materials. Further innovations in flash sintering have focused on improving current pathways and enhancing localized heating effects. Lu et al. [126] developed a method to create thermal pyrolytic carbon (PyC) “bridges” between SiC particles through the carbonization of phenolic resin. These PyC bridges provided abundant conductive pathways, reducing the sintering time and facilitating rapid densification. The transformation of PyC from amorphous carbon to oriented graphite carbon during the sintering process indicated the successful generation of localized ultra-high-temperature environments. This technique also shows potential for localized repair of matrix damage in SiC ceramic-based composites, further expanding the applicability of flash sintering technology (Figure 16). Flash sintering offers several advantages, including reduced energy consumption, shorter production cycles, and lower furnace temperatures, making it one of the most advanced energy-efficient technologies for densifying SiC ceramics [127]. However, despite these promising developments, several aspects of the underlying sintering mechanisms remain poorly understood. Key uncertainties include the precise roles of the applied current and electric field during the flash phenomenon, the mechanisms governing material transport and densification, and the chemical reactions occurring within the ceramic matrix. These knowledge gaps limit the broader adoption and optimization of flash sintering for industrial applications. Future research should focus on elucidating the fundamental mechanisms of flash sintering, particularly the interplay between electrical and thermal effects during densification. Additionally, developing advanced models to predict the evolution of microstructures and properties under flash sintering conditions will be critical for process optimization. Efforts to integrate flash sintering with other advanced techniques, such as additive manufacturing or hybrid sintering methods, could further expand its industrial applications. By addressing these challenges, flash sintering has the potential to revolutionize the fabrication of SiC ceramics, offering a sustainable and efficient pathway for producing high-performance materials in fields such as aerospace, energy, and electronics.